I've never used the AppImage Package Manager, but if you can set it up to run unattended, then by all means, go for it.

Also, from what I’ve read in the Topgrade documentation, this program can handle everything that the sysupdate.sh script does, as well as AppImage (and much more).
You can then add Topgrade to your cron schedule instead of the sysupdate.sh script.
And if you need to run Topgrade manually as an ordinary user, you can execute it with the command:

su -c /usr/bin/topgrade

It's not too difficult. Save the contents to a file, such as sysupdate.sh, and set its execution permissions:

# chmod a+x sysupdate.sh

Then move the file to the /usr/bin directory:

# mv ./sysupdate.sh /usr/bin

Next, edit the cron configuration file:

# nano /etc/crontab

and enter the command to run the sysupdate.sh script:

0 21 * * * root /usr/bin/sysupdate.sh

This entry means that cron runs the script every day at 21:00 as root. Of course, you can change the script’s execution time to whatever you need.

Alternatively, instead of messing with editing crontab, after having granted the script execute permissions and moving it to /usr/bin, you can make a symbolic link in the /etc/cron.daily directory

# ln -s /usr/bin/sysupdate.sh /etc/cron.daily

and the script will be executed once a day.

That’s it. The script runs on its own, updates the repositories, installs new package versions, and cleans the system of old or unnecessary versions. For system-critical packages, changes take effect after the system reboot. This isn’t a problem for me, as I’m not a fan of keeping the system run for months.

In the script itself, you can block, for example, updates to Flatpak applications if you don’t have them.

The section at the very beginning of the script (5 lines below the #!/bin/bash line) is for systems with sudo and forces the sudoers to enter their password when the script is run manually by the user. In Devuan, you can ignore this and run the script manually with the command

su -c sysupdate.sh